We will investigate the structures and the mechanisms of three different enzymes that catalyze similar lyase reactions of citrate. We will isolate for comparison one of these enzymes from many sources as well as isolating pairs of these enzymes from a single source. These comparative studies will aid both in the understanding of each mechanism, the evolution of a single enzyme and the evolution of an active site. Comparison of the different enzyme structures and their similar functions will aid in the general problem of structure-function of proteins.